perf(discover): 剧场与新片页网格改为按行懒加载

双列网格原来是两个 Column 一次性建出所有卡片，翻几页后上百张卡连同封面都留在树上；
现在改成按行的 SliverList，只构建可见行，剧场页/新片页/搜索结果的滚动容器一并换成 sliver。
搜索结果、加载失败等状态同步改成 sliver，滚动位置与下拉刷新行为保持不变。

lib/pages/discover/discover_view.dart

@@ -153,14 +153,19 @@ class _DiscoverPageState extends State<DiscoverPage> {
               edgeOffset: _stickyHeaderHeight + 8,
               onRefresh: () =>
                   Future.wait([categories.load(), theater.refreshCurrent()]),
-              child: ListView(
+              child: CustomScrollView(
                 controller: logic.scroll,
                 physics: AppPullToRefresh.physics,
-                padding: EdgeInsets.only(
-                  top: showHeader ? _stickyHeaderHeight : 0,
-                  bottom: AppDimens.spaceXl,
-                ),
-                children: _content(l10n, categories),
+                slivers: [
+                  if (showHeader)
+                    const SliverToBoxAdapter(
+                      child: SizedBox(height: _stickyHeaderHeight),
+                    ),
+                  ..._content(l10n, categories),
+                  const SliverToBoxAdapter(
+                    child: SizedBox(height: AppDimens.spaceXl),
+                  ),
+                ],
               ),
             ),
           ),
@@ -228,12 +233,12 @@ class _DiscoverPageState extends State<DiscoverPage> {
       ];
     }
     if (categories.loading && categories.categories.isEmpty) {
-      return [_block(const CircularProgressIndicator(strokeWidth: 2))];
+      return [_sliverBlock(const CircularProgressIndicator(strokeWidth: 2))];
     }
     final error = categories.error;
     if (error != null) {
       return [
-        _block(
+        _sliverBlock(
           EmptyState(
             key: const Key('home-categories-error'),
             icon: Icons.cloud_off_rounded,
@@ -253,12 +258,12 @@ class _DiscoverPageState extends State<DiscoverPage> {
       ];
     }
     if (theater.loadingFirstPage) {
-      return [_block(const CircularProgressIndicator(strokeWidth: 2))];
+      return [_sliverBlock(const CircularProgressIndicator(strokeWidth: 2))];
     }
     final theaterError = theater.lastError;
     if (theaterError != null && theater.dramas.isEmpty) {
       return [
-        _block(
+        _sliverBlock(
           EmptyState(
             key: const Key('home-categories-error'),
             icon: Icons.cloud_off_rounded,
@@ -282,38 +287,49 @@ class _DiscoverPageState extends State<DiscoverPage> {
   List<Widget> _theaterBlocks(AppLocalizations l10n) {
     final dramas = theater.dramas;
     return [
-      _toolbar(),
+      SliverToBoxAdapter(child: _toolbar()),
       if (dramas.isEmpty)
-        Padding(
-          padding: const EdgeInsets.symmetric(vertical: 40),
-          child: EmptyState(
-            key: const Key('discover-empty'),
-            icon: Icons.movie_filter_outlined,
-            title: l10n.noStories,
-            subtitle: l10n.emptyHint,
+        SliverToBoxAdapter(
+          child: Padding(
+            padding: const EdgeInsets.symmetric(vertical: 40),
+            child: EmptyState(
+              key: const Key('discover-empty'),
+              icon: Icons.movie_filter_outlined,
+              title: l10n.noStories,
+              subtitle: l10n.emptyHint,
+            ),
           ),
         )
       else
-        DramaGrid(l10n: l10n, dramas: dramas, onOpen: _openDrama),
+        SliverDramaGrid(
+          key: const Key('home-all-dramas-grid'),
+          l10n: l10n,
+          dramas: dramas,
+          onOpen: _openDrama,
+        ),
       if (theater.loadingMore)
-        const Padding(
-          padding: EdgeInsets.symmetric(vertical: 20),
-          child: Center(
-            child: SizedBox.square(
-              key: Key('theater-load-more'),
-              dimension: 22,
-              child: CircularProgressIndicator(strokeWidth: 2),
+        const SliverToBoxAdapter(
+          child: Padding(
+            padding: EdgeInsets.symmetric(vertical: 20),
+            child: Center(
+              child: SizedBox.square(
+                key: Key('theater-load-more'),
+                dimension: 22,
+                child: CircularProgressIndicator(strokeWidth: 2),
+              ),
             ),
           ),
         )
       else if (dramas.isNotEmpty && !theater.hasMore)
-        Padding(
-          key: const Key('paged-list-no-more'),
-          padding: const EdgeInsets.symmetric(vertical: 16),
-          child: Text(
-            l10n.noMoreItems,
-            textAlign: TextAlign.center,
-            style: const TextStyle(color: AppColors.textHint, fontSize: 12),
+        SliverToBoxAdapter(
+          child: Padding(
+            key: const Key('paged-list-no-more'),
+            padding: const EdgeInsets.symmetric(vertical: 16),
+            child: Text(
+              l10n.noMoreItems,
+              textAlign: TextAlign.center,
+              style: const TextStyle(color: AppColors.textHint, fontSize: 12),
+            ),
           ),
         ),
     ];
@@ -401,6 +417,8 @@ class _DiscoverPageState extends State<DiscoverPage> {
     Navigator.of(context).push<void>(MaterialPageRoute(builder: (_) => page));
   }
 
+  Widget _sliverBlock(Widget child) => SliverToBoxAdapter(child: _block(child));
+
   Widget _block(Widget child) => ConstrainedBox(
     constraints: const BoxConstraints(minHeight: 280),
     child: Center(child: child),